Communication systems that geographically reuse communication channels are known These systems allocate a predetermined set of communication channels in one geographic area and reuse the same set of communication channels in one or more other geographic areas. This reuse technique improves communication capacity by minimizing the number of communication channels necessary to provide communication service in a large geographic area comprised of several smaller geographic areas. As is also known, a communication channel may be a frequency carrier or a pair of frequency carriers in a frequency division multiple access (FDMA) communication system, a time slot or a pair time slots in a time division multiple access (TDMA) communication system, or any other radio frequency (RF) transmission medium.
Two of the most common communication systems that geographically reuse communication channels are cellular communication systems and trunked mobile communication systems. In both communication systems, allocation of a communication channel begins when a communication unit requests communication service. Based on channel availability and signal quality, a system controller assigns the communication channel to the communication unit A communication, such as a conversation or a facsimile transmission, occurs on the communication channel between the communication unit and another communication unit or between the communication unit and a subscriber to a public service telephone network. The communication continues until completion or an interruption in service occurs. Upon conclusion of the communication, the system controller retrieves the communication channel, thereby making the communication channel available for another communication.
An important parameter in identifying an acceptable communication channel is signal quality. In a wireless communication system, the communication channels are typically RF channels which occupy predetermined bandwidths. When information signals are transmitted on the RF channels, undesired channel effects, such as fading, interference and noise, alter the information signals during transmission. Thus, the information signals received by a receiver in the communication unit, or a base station, are corrupted by the undesired channel effects. By ascertaining an indication of the corruption on available communication channels, the least corrupted communication channel may be selected for the communication. This indication of corruption is known as signal quality.
In geographic reuse communication systems, signal quality is typically limited by the quantity of co-channel interference present on the RF channel. Co-channel interference occurs when receivers receive unwanted information signals from neighboring communication units, or base stations, transmitting on the same RF channel as the desired RF channel. Thus, signal quality decreases as co-channel interference increases.
Received signal strength indication (RSSI), bit error rate (BER), and carrier-to-interference plus noise ratio are three common methods of estimating signal quality. In an RSSI estimate, the receiver measures the level of a received signal on the desired RF channel. This measurement provides a summation of signal levels including the desired information signal (C), the co-channel interference (I), and the noise (N) on the desired RF channel. Accordingly, RSSI does not distinguish between the desired information signal level and the level of undesired channel effects. In a BER measurement, the receiver measures the number of errors detected in a received bit stream BER measurements provide accurate estimates of signal quality in high error rate environments, but in geographic areas where error rates are low, multiple measurements and excessive averaging times may be required to obtain the accurate estimates. In a carrier-to-interference plus noise ratio determination, the receiver isolates the desired information signal (C) from the summation of co-channel interference and noise (I+N) and forms a ratio of the two quantities (C/(I+N)) to more accurately evaluate the quality of the communication channel used to convey the desired information. One approach to determining carrier-to-interference plus noise ratios in a digital communication system is described in U.S. Pat. No. 5,440,582, entitled "Method and Apparatus for Determining Signal Usability" and assigned to Motorola, Inc.
Presently, the three common methods of determining signal quality of a communication channel make such a determination by evaluating the communication channel as a whole. For example, if a TDMA system permits a voice user to transmit in only one time slot per time frame, then the transmission channel for each voice user is one slot per time frame. Accordingly, to measure the signal quality of such a transmission channel, the receiver in the TDMA system measures the signal quality of the assigned slot in each time frame to determine the overall channel signal quality. However, current communication systems are embarking on efforts to improve received audio quality by increasing the amount of digitally encoded voice transmitted in a particular time period. In such systems, two or more time slots per time frame may be necessary to convey the encoded voice. In addition, these new systems may be dual-mode, simultaneously permitting improved audio quality transmission for one user in two slots per time fame (e.g., in a telephone-interconnect mode) and standard audio quality transmission for another user in one slot per frame (e.g., in a dispatch mode).
In such dual-mode systems, existing signal quality measurement approaches (i.e., measuring one slot per time frame) are inefficient for obtaining accurate channel signal quality for improved audio quality transmissions because a standard audio quality transmission in one slot per time frame may be interfering with an improved audio quality transmission in two slots per time frame. Such interference from the standard audio quality transmission is detectable only if the receiver receiving the improved audio quality transmission is measuring the signal quality of the particular time slot of the two slot improved audio channel that contains the interfering standard audio transmission. If the receiver of the improved audio quality transmission is not measuring the signal quality of the correct time slot of the two slot channel, the receiver will, in most cases, determine an errantly higher or better channel signal quality.
Therefore, a need exists for a method of determining signal quality of a communication channel that accurately detects signal quality in the aforementioned dual-mode type of communication system. Such a method that further permits handoff decisions to be made based on such a signal quality determination would be an improvement over the prior art.